Devices employing liquid crystals have found use in a variety of electrooptical applications, in particular those which require compact, energy-efficient, voltage-controlled light valves, e.g., watch and calculator displays, as well as the flat-panel displays found in portable computers and compact televisions. Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which make them the most promising of the non-emissive electrooptical display candidates currently available.
A recent advance in the liquid crystal art has been the utilization of tilted chiral smectic liquid crystals, one class of which are termed ferroelectric liquid crystals, in devices which give microsecond switching and bistable operation. Ferroelectric liquid crystals were discovered by R. B. Meyer et al. (J. Physique 36, 1-69 (1975).). A high speed optical switching phenomenon using a "surface-stabilized ferroelectric liquid crystal" (SSFLC) was discovered for the ferroelectric liquid crystals by N. A. Clark et al. (Appl. Phys. Lett. 36, 899 and U.S. Pat. No. 4,367,924).
Many new ferroelectric liquid crystals have been developed and their switching characteristics extensively tested. Although devices employing these materials exhibit high response speed and wide viewing angles, many problems remain in developing SSFLC devices. These problems have included insufficient threshold characteristics, unsatisfactory contrast (due to chevron defects), and insufficient bistability due to difficulties in controlling alignment.
More recently, antiferroelectric liquid crystals (AFLC), another class of tilted chiral smectic liquid crystals, have been developed. Antiferroelectric liquid crystals are switchable in a chiral smectic C.sub.A phase (SC.sub.A * phase) in addition to the tilted chiral smectic C phase (S.sub.C * phase) used in ferroelectric liquid crystal devices.
One of the most important characteristics of a liquid crystal display device is its response time, i.e., the time required for the device to switch from the on (light) state to the off (dark) state. Especially important for practical operation of larger devices is the response time with respect to temperature. In such devices, temperature non-uniformities may adversely effect the performance and require some form of compensation, unless the switching speed is largely independent of temperature. In a ferroelectric or anti-ferroelectric device, response time .tau..sub.electric is proportional to the rotational viscosity (.eta.) of the liquid crystal compound(s) contained within the device and is inversely proportional to their polarization (P.sub.s) and to the applied electric field (E) according to the following formula: EQU .tau..sub.electric =.eta./P.sub.s E.
Thus, response time can be reduced by using compound(s) having high polarizations or low viscosities, and such compounds are greatly desired in the art. In addition, compounds with polarizations that rapidly increase with decreasing temperature can lead to temperature independence or reduced temperature dependence of switching.
In the passive addressing of liquid crystal compounds exhibiting a spontaneous polarization, however, low polarization mixtures may be preferred for the practical operation of a liquid crystal device. Polarization reversal fields are larger for higher polarization mixtures, and polarization reversal fields cause switching or partial switching back to a material's original director alignment. This results in loss of the bistability that is crucial to the passive-matrix driving of ferroelectric liquid crystal devices.
Another potential disadvantage of using high polarization mixtures is the partial switching of their director alignment in response to non-switching (secondary) signals in a driving waveform. This continued response or fluctuation of the director causes a large decrease in the contrast ratio of a ferroelectric liquid crystal device.
There remains a need in the art for liquid crystal materials having fast response times, ideally possessing broad smectic temperature ranges to enable operation of the device over a broad range of temperatures, or should be capable of combination with other liquid crystal compounds having different smectic temperature ranges without adversely affecting the smectic phase behavior of the base mixture. There further remains a need in the art for materials which have low polarization values, and which can provide reduced temperature dependence of the response time. Further, there remains a need in the art for novel liquid crystal materials which exhibit tristable switching.